This can occur when using either POP3 or SMTP inbound to the K1000. This issue typically occurs due to sender verification failure.
Sender verification failure most commonly occurs due to the use of split-brain (aka split-horizon, split-view, or split) DNS in the K1000's environment. Split-brain DNS occurs when the same DNS zone (e.g. domain.com) has an authoritative zone internally and externally.
Whether or not split-brain DNS is the cause, MX records should be validated as the first step of troubleshooting. The following Windows-based client test can determine whether the issue is caused by missing MX records.
Note: This test must be performed as specified against the DNS server configured in K1000 Network Settings. External MX records for a split-brain DNS zone do not negate the possibility of internal DNS zone issues.
From a client on the same internal network as the K1000:
Test output should look something like this:
If you do not see any MX records listed, then sender verification will fail for that domain. This results in all email from the domain in question being dumped by the K1000 since it will not process emails that fail sender verification - which is a SPAM prevention technology used in many email server technologies.
If you do see MX records listed, then the hostname for the MX record must resolve to the IP address of the server sending mail to the K1000 from that domain. If this is not true, then email for this domain from this mail server will be dumped similarly as if no MX record existed at all. Valid Example: 1. MX record exists for mail.domain.com; 2. mail.domain.com resolves to 192.168.2.10; 3. 192.168.2.10 is the mail server sending on behalf of domain.com to the K1000.